1. Field of the Invention
This invention relates, in general, to electrical apparatus and, more specifically, to insulating structures for electrical transformers.
2. Description of the Prior Art
Insulation is required for electrical inductive apparatus, such as transformers, to insulate the electrical conducting elements from each other and from ground and, thereby, give the transformer the ability to withstand normal and overload conditions and, also, impulse voltages. Cellulosic materials, such as kraft paper and pressboard, have been employed in liquid-filled transformers due to their excellent dielectric properties and low cost. However, the physical and electrical properties of these cellulostic materials deteriorate at an increasing rate when the operating characteristics of the transformer rise above 100.degree. C thereby limiting the rating and the average operating temperature of transformers employing these materials due to their thermal instability. Indeed, cellulostic materials cannot be used in certain transformer constructions, such as those employing a liquid which vaporizes within the normal operating temperature range of the transformer to provide cooling and insulation therefor, since such transformers operate at a temperature considerably higher than the average temperature in conventional liquid-filled transformers thereby eliminating cellulostic materials as a viable insulation alternative.
In such transformers or even liquid-filled transformers having a rating and average temperature above the allowable range of cellulostic materials, an insulative material comprised of wholly aromatic polyamide fibers, such as one sold commercially under the trade name "NOMEX", has proved effective. As explained in greater detail in U.S. Pat. No. 3,585,552, issued to L. Feather, the use of such material in the insulating structures provides a substantial increase in insulative qualities over cellulostic materials. However, insulating structures comprised solely of wholly aromatic polyamide fibers are quite costly. Thus, it would be desirable to provide an insulating structure for an electrical inductive apparatus, such as a transformer, which provides substantially the same insulating properties as a paper formed of wholly aromatic polyamide fibers, but at a reduced cost.
Other dielectric films, such as polyester films, have been thought to be unsuited for use as solid insulation in most transformer applications as the electrical impulse strength of these materials drops rapidly when used in thicknesses above 5 mils. In addition, such films embrittle quickly at temperatures above 200.degree. C and, also, exhibit plastic flow at the temperatures and pressures experienced during a short circuit of a transformer. However, the polyester films are relatively inexpensive when compared to papers formed of wholly aromatic polyamide fibers and, furthermore, have a higher dielectric strength in the non-condensable gas environment commonly used in vaporization cooled transformers.
Thus, it would be desirable to provide an insulating structure for certain electrical apparatus, such as electrical transformers, that enables the inexpensive polyester films to be used in such a manner so as to take advantage of certain of their superior electrical characteristics while, at the same time, minimizing the effects of their less advantageous properties.